1. Field of the Invention
The present invention generally relates to a differential amplifier, and more particular, to a differential amplifier with a plurality of input pairs.
2. Description of Related Art
Along with the prosperous progress of electronics, a differential amplifier has being broadly used in various circuit designs. A typical differential amplifier comprises two input terminals, and the output signal thereof is the difference between two voltages at the two input terminals multiplied by the gain of the amplifier. In order to switch different input signals, a conventional differential amplifier employs switches at the input terminals thereof to accomplish the switch operations.
FIG. 1 is a circuit diagram of a conventional differential amplifier to switch two input pairs. Referring to FIG. 1, the differential amplifier includes a load unit 101, a load unit 103, two transistors 105 and 107, a current source 109 and switches 111, 113, 115 and 117. The operation voltage and the grounded terminal herein are respectively represented by VDD and GND. As the switch 111 is turned on, an input signal INA1 of a first input pair is conducted to the gate terminal of the transistor 105; as the switch 113 is turned on, an input signal INB1 of the first input pair is conducted to the gate terminal of the transistor 107. Thus, as the switches 111 and 113 are simultaneously turned on, the differential amplifier would receive the input signals of the first input pair.
Similarly, as the switch 115 is turned on, an input signal INA2 of a second input pair is conducted to the gate terminal of the transistor 105; as the switch 117 is turned on, an input signal INB2 of the second input pair is conducted to the gate terminal of the transistor 107. Thus, as the switches 115 and 117 are simultaneously turned on, the differential amplifier would receive the input signals of the second input pair. It can be seen that in the prior art, the input signals of a plurality of input pairs are switched by using a plurality of switches disposed at the gate terminals of the transistors 105 and 107.
FIG. 2 is a circuit diagram of another conventional differential amplifier to switch two input pairs. Referring to FIG. 2, the differential amplifier includes a load unit 101, a load unit 103, two transistors 105 and 107, a current source 109, a load unit 201, a load unit 203, two transistors 205 and 207, a current source 209 and switches 211, 213, 215 and 217. It can be seen from FIG. 2 that the prior art employs an additional differential amplifier to provide another set of input signals incorporated with four switches to achieve the switching operations of a plurality of input pairs. In other words, as the switches 211 and 213 are turned on, the output signals OUTA and OUTB are provided by the input signals INA1 and INB1 of the first input pair; as the switches 215 and 217 are turned on, the output signals OUTA and OUTB are provided by the input signals INA2 and INB2 of the second input pair. It can be seen that in the prior art, the input signals of a plurality of input pairs are switched by disposing additional differential amplifiers and using a plurality of switches disposed at the output terminals thereof.
The differential amplifiers shown by FIGS. 1 and 2 have a common disadvantage that the turn-on impedance of the switches is too high, which would largely affect the quality of high-frequency signals. In order to lower the turn-on impedance, the switch area must be substantially increased. In addition, the switches 211, 213, 215 and 217 must be disposed at the output nodes as shown by FIG. 2, but in fact, the two differential amplifiers may be located quite far from each other and long signal lines may needed to connect the two sets of output signals to the switches. Thus, the long signal lines may become one of electromagnetic interference sources and lead to poor signal quality. Furthermore, considering that in the prior art, metal oxide semiconductor transistors (MOS transistors) are popularly used as switches, for example, as shown in FIG. 1, four switches are disposed at the input signal terminals of the differential amplifier. Thus, additional electrostatic discharge devices (ESD device) are required, which are a considerable waste of space and cost. In FIG. 2, although the additional ESD device is saved, but an additional differential amplifier is required, wherein in particular, a load unit is employed, which leads a significant increase in cost.
Accordingly, manufacturers are eager to seek a proper solution to overcome the above-mentioned problems.